Small cell is one of the hottest topics in long-term evolution advanced (LTE-A) Release 12.
Currently, the most of operating cost of mobile communication operators is the energy charge which is expended by base stations. Some researches show that energy can be effectively saved by replacing a macro cell with small cells on the premise of the same covering area, because base stations of the small cells are of small power, produce less heat, and may need fewer or may not need cooling devices. Besides, the size of small cell base stations is much smaller than that of macro cells. Small cells are of low cost and are easy to be deployed.
In some cases, such as in an indoor environment, a channel from a base station of the small cells to user equipment (UE) contains a line-of-sight, which is less subjected to interference in transmission, and a signal to noise ratio in transmission is relatively high. A good signal propagation environment can ensure correct transmission of modulation symbols of higher orders, thereby efficiently improving spectrum efficiency. Hence, LTE of next release will support a constellation modulation scheme of a higher order, such as 256 QAM modulation.
In order to better support adaptive coding modulation transmission, LTE supports flexible code rate and modulation scheme. First of all, UE calculates corresponding PMI (precoding matrix indicator) and RI (rank indication) information based on an estimation result of channels, and selects suitable CQI(Channel quality indicator) indices for a wide band or a subband according to its own abilities of reception and demodulation, and then feeds the above information back to the base station in an agreed manner in LTE. LTE supports 16 CQI indices are supported as shown in Table 1 (which is Table 7.2.3-1 in 3GPP TS 36.213). Besides that CQI index 0 is used to denote “out of range”, other 15 CQI indices correspond respectively to a coding and modulation scheme, that is, a modulation scheme and a code rate of channel coding. The UE uses uplink transmission to feed back the CQI indices obtained through calculation to the eNB. The CQI index of the wide band uses that an absolute transmission scheme is carried by 4-bit information. CQI information (containing a PMI, RI and a CQI index) may be carried and transmitted by a physical uplink control channel (PUCCH) or a physical uplink shared channel (PUSCH).
TABLE 14-bit CQI tableCQI Code indexModulationrate × 1024Efficiency0Out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
After obtaining the CQI information fed back by the UE, the base station schedules and configures the UE based on a realtime scheduling and load situation and the information fed back by the UE. The base station will indicate time and frequency resources where downlink data transmitted to the UE are located and a coding and modulation scheme employed in transmission to the UE via a physical downlink control channel (PDCCH). Or the eNB indicates the UE via a PDCCH which time and frequency resource and which coding and modulation scheme should be employed for uplink data transmission. The coding and modulation scheme is indicated via 5-bit information (IMCS) here. Except PDCCH format 3A (which is specifically used for power control indication of an uplink control channel), all other PDCCH formats contain one or two pieces of IMCS information. The number of pieces of IMCS information is related to the number of transport blocks (TBs) in transmission, and there is at least one TB and at most two TBs in one time of transmission.
After having received the IMCS information, the UE looks up tables to get a an ITBS index and a modulation scheme. If the received IMCS is used for downlink transmission demodulation decoding, Table 2 (which is Table 7.1.7.1-1 in 3GPP specification TS 36.213) is looked up. If the received IMCS is used for uplink transmission coding modulation, Table 3 (which is Table 8.6.1-1 in 3GPP specification TS 36.213) is looked up.
TABLE 2Modulation and TBS index table for PDSCHMCS IndexModulation OrderTBS IndexIMCSQmITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
TABLE 3Modulation, TBS index and redundancy versiontable for PUSCHMCS ModulationTBS RedundancyIndexOrderIndexVersionIMCSQ′mITBSrνidx020012102220323042405250626072708280929010210011410012411013412014413015414016415017416018417019418020419021619022620023621024622025623026624027625028626029reserved1302313
Thereafter, the UE may obtain a size of an information bit of a TB(resource block) according to the ITBS and the size of the transmission resource (the number of resource blocks (RBs)) by looking up the TBS(transport block size) table, so as to indirectly obtain the code rate. As an example, Table 4 only gives a TBS table to which 1-10 RBs correspond (from Table 7.1.7.2.1 in 3GPP specification TS 36.213); where, NPRB is the number of the RBs.
TABLE 4TBS table (size: 27 × 110)NPRBITBS1234567891001632568812015217620822425612456881441762082242563283442327214417620825629632837642434010417620825632839244050456845612020825632840848855263269657214422432842450460068077687263281762563925046007128089361032710422432847258471284096810961224812025639253668080896810961256138491362964566167769361096125614161544101443285046808721032122413841544173611176376584776100011921384160818002024122084406809041128135216081800202422801322448874410001256154418002024228025361425655284011281416173619922280260028561528060090412241544180021522472272831121632863296812881608192822802600298432401733669610641416180021522536285632403624183767761160154419922344279231123624400819408840128817362152260029843496388042642044090413841864234427923240375241364584214881000148019922472298434964008458449682252010641608215226643240375242644776535223552112817362280285634964008458451605736245841192180024082984362442644968554459922561612561864253631123752439251605736620026712148022162984375243925160599267127480
It can be seen from the above description that the CQI index and the IMCS are very important indication information in uplink and downlink transmission of an LTE system. The CQI index is carried by 4-bit information, and may indicate at most sixteen cases currently, each case having a corresponding coding and modulation scheme. And the IMCS is carried by 5-bit information, and may indicate at most thirty-two cases currently, whatever uplink or downlink, each case having a corresponding coding and modulation scheme. If a new modulation scheme, such as 256 QAM, is added, 1 bit must be added to both the CQI index and the IMCS; otherwise, following a current manner cannot indicate a coding and modulation scheme that is modulated by using 256 QAM.
However, in the implementation of the present disclosure, the inventors found that in an existing transmission manner, if 1 bit is added to both CQI index and IMCS, about 10% of the load will be increased in each time of transmission. This may possibly lower transmission reliabilities of related uplink and downlink control information. For example, in a case that physical transmission resources are unchanged, addition of uncoded information will reduce coding check redundancy bits, thereby lowering protection force of coding for the information.
It should be noted that the above description of the background art is merely provided for clear and complete explanation of the present disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background art of the present disclosure.